1. Field of the Invention
The present invention relates to a wafer flattening process for etching and flattening projecting portions of a wafer surface locally by an activated species gas or locally etching relative thick portions of a wafer to achieve a uniform distribution of thickness of the wafer and to a system for the same.
2. Description of the Related Art
FIG. 11 is a schematic cross-sectional view of an example of a wafer flattening process of the related art.
In FIG. 11, reference numeral 100 is a plasma generation unit. Activated species gas G in the plasma generated by the plasma generator unit 100 is sprayed on the front surface of a wafer W from a nozzle 101.
The wafer W is placed and fixed on a stage 120. The stage 120 is made to move in the horizontal direction to guide a portion relatively thicker than a prescribed thickness on the front surface of the wafer W (hereinafter referred to as a "relatively thick portion") directly under the nozzle 101.
The activated species gas G is then sprayed from the nozzle 101 to the projecting relatively thick portion Wa to locally etch the relatively thick portion Wa and achieve a uniform distribution of thickness of the front surface of the wafer W.
The thickness of the relatively thick portion Wa of the wafer W is not however uniform but is diverse.
Therefore, a technique has been devised for controlling the relative speed of the nozzle 101 with respect to the wafer W to match with the thickness of the relatively thick portion Wa (for example, the technology disclosed in Japanese Patent Laid-Open No. 9-27482).
This technique calls for measuring the positions and thicknesses of relatively thick portions Wa over the entire surface of the wafer W by a wafer flatness measurement apparatus to create two-dimensional position-thickness data. This data is converted to position-relative speed data showing the positions of the relatively thick portions Wa and the relative speeds of the nozzle 101 for making the relatively thick portions Wa a desired flatness after the etching.
Next, the stage 120 is controlled based on the position-relative speed data to make the nozzle 101 directly over predetermined relatively thick portions Wa to etch the entire surface of the wafer W.
That is, at a relatively thick portion Wa with a large thickness, the relative speed of the nozzle 101 is reduced to increase the amount of etching, while at a relatively thin portion Wa with a small thickness, the relative speed of the nozzle 101 is increased to reduce the amount of etching so as to thereby flatten the entire surface of the wafer W.
In the above wafer flattening process of the related art, however, there were the following problems.
Since the ions in the plasma generated at the plasma generation unit 100 are accelerated by the potential difference applied between the plasma and the wafer W and strike the wafer W, just the portions which the ions strike are etched to a large degree. Further, the atoms of the surface of the wafer W are removed by the sputtering. Therefore, the surface of the wafer W is roughened on an atomic order.
Further, the particles floating around the wafer W and the particles generated in the discharge tube forming the nozzle 101 deposit on the front surface of the wafer W. The etching characteristics of the portions where the particles are deposited decline. As a result, the amounts of etching of the portions where the particles are deposited and the portions where they are not deposited become different and the front surface of the wafer W becomes rough.
Due to the above reasons, local etching ends up resulting in a larger mean squared roughness (hereinafter referred to as the "IRMS") of the front surface of the wafer W. When the front surface of the wafer W after the local etching is observed by an interatomic microscope, it is seen that when a wafer W with an RMS before local etching smaller than 1 nm is locally etched by the above wafer flattening process, the RMS ends up deteriorating about 10 nm.